Rocket engines depend on hydrogen for fuel and require high strength, tough materials which are not embrittled by the hydrogen. At the leading edge of technology in this field, the present Space Shuttle Main Engine (SSME) has been designed for orbital space flight and has developed approximately a one-half million pound sea level thrust. Its turbines operate at extremely high speed and high pressure using hydrogen and steam as the working fluids. The turbine blades of the fuel turbo pumps are subjected to high alternating stresses together with extreme thermal transients. It is therefore crucial that the alloy employed in the SSME be one which is capable of withstanding these extreme conditions.
At present, the alloy used in the Space Shuttle Main Engine and other rocket engines is an alloy known as MAR-M-246(Hf), which is in the directionally-solidified and heat treated condition. This alloy was originally designed for gas-turbine engines, and has been adapted for use in the turbine blades of the SSME turbo pump, even though the operating conditions of rocket engines are different from those of the gas-turbine engines. This alloy has been able to meet the initial structural requirements, but is somewhat limited in life. Further, the initial strength of the material is only 60% in hydrogen when compared to air, and notch strength ratio is 18%. The MAR-M-246(Hf) is a multi-phase polycrystalline alloy, and the behaviors of these phases are vastly different due to their individual characteristics. Most of these phases deteriorate under the extremely demanding service conditions. For example, carbides, which are employed in the alloy and originally are of small size, will tend to coagulate, becoming large enough to be potential centers of stress under the rigorous pressure and temperature conditions. They will eventually be responsible for initiating and propagating cracks which may ultimately cause failure.
It is also important to note that the carbide-formers have been intentionally added to strengthen the grain boundaries against creep and grain growth phenomena at high temperatures. However, most of these carbide-formers are known to lower the solvus temperature of the gamma-prime phase, thereby drastically reducing its most beneficial effect as the effective strengthener of the matrix. It is thus almost impossible to control or inhibit the diverse changes in all of the various phases at the same time. The depletion of alloying elements will precede at different rates in different phases and will cause severe loss in strength in many areas. Moreover, the grain boundary and the gamma/gamma-prime interface areas, being high energy areas, will be highly susceptible to environmental degradation by attracting hydrogen to these regions. The result will be a deterioration by embrittlement of this alloy, thereby shortening its effective lifetime.
A more desirable alloy should have fewer phases, and these phases should be compatible and controllable in a structural sense. The alloy should not contain any grain boundary or undesirable phase such as the topologically close packed (TCP) phases. The alloy used in the fuel turbine blade of rocket engines must be resistant to hydrogen in addition to having high strength, good fatigue characteristics, and good creep rupture strength at elevated temperatures. All known present superalloys are incapable of meeting the rigorous demands for repeated space flights even of short duration.